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The recycling of polyurethane materials
Time:2020/8/5 15:55:40    

In recent years, polyurethane products have become more and more popular among people. For example, they are widely used in refrigerator insulation, mattresses, and auto parts in daily life.

In recent years, polyurethane products have become more and more popular among people. For example, they are widely used in refrigerator insulation, mattresses and auto parts in daily life. Although polyurethane has always been labeled as environmentally friendly and recyclable, in fact, the recycling of polyurethane materials is not optimistic after the final product is made, and it is still one of the problems that plague this industry.

Plants producing polyurethane foam produce a large amount of scraps, mold spills, and waste products every year and a large amount of waste is also generated in various downstream applications, such as old polyurethane foam and elastomers in scrapped cars which need to be recycled.

The landfill method is a helpless method which not only takes up land but also consumes costs.Since polyurethane is a nitrogen-containing polymer, the burning conditions must be strictly controlled to recover heat energy from incineration. An absorption device is also required to reduce environmental pollution from toxic gases.In recent years, through the gradual exploration of scientific researchers, polyurethane recycling methods have also begun to enrich. This article will introduce some physical and chemical methods of recycling waste polyurethane, hoping to give you a more comprehensive perspective.

 

Physical Recycling

Physical recycling provides an effective and economical method for the recycling of rigid polyurethane foam and composite materials. First, reduce the particle size of the original polyurethane foam product to the standard that can be reprocessed during the secondary processing. Waste recyclables or leftovers generated during the production process are degraded into more useful forms, such as flakes, granules or powder.

1. Bonding and forming

This method is the most widely used recycling technology. Through the pulverizer, the flexible polyurethane foam is pulverized into pieces of several centimeters and the reactive polyurethane adhesive is sprayed in the mixer.The adhesive used is generally polyurethane foam composition or NCO-terminated prepolymer based on polyphenyl polymethylene polyisocyanate (PAPI).When the PAPI-based adhesive is used for bonding and molding, it can also be mixed with steam.In the process of bonding waste polyurethane, 90% waste polyurethane and 10% adhesive are added and mixed evenly. You can also add some dyes and then pressurize the mixture.

The bonding molding technology not only has great flexibility, but also has great variability in the mechanical properties of the final product.The most successful recycling method for polyurethane products is to use waste foam such as soft foam scraps to produce recycled polyurethane foam through bonding methods, which are mainly used as carpet backing, sports mats, sound insulation materials and other products.Under certain temperature and pressure, soft foam particles and adhesives can be molded into products such as car bottom pads; with higher pressure and temperature, hard parts such as machine pump shells can be molded out.

Rigid polyurethane foam, reaction injection molding (RIM) polyurethane elastomer, etc. can also be recycled in the same way. The waste particles are mixed with the isocyanate prepolymer and formed by hot pressing, for example, to manufacture the pipe bracket of the pipe heating system.

2. Hot pressing

Thermosetting polyurethane soft foam and RIM polyurethane products have certain thermal softening and plastic properties in the temperature range of 100-200℃. Under high temperature and high pressure, waste polyurethane can be bonded to each other without using adhesives. In order to make the recycled products more uniform, the waste is often crushed and then heated and pressurized to form.

The molding conditions depend on the type of waste polyurethane and recycled products. For example, flexible polyurethane foam can be made into parts such as shock absorbers, mudguards, etc. by hot pressing at a pressure of 1~30MPa and a temperature range of 100~220°C for several minutes.

This method has been successfully applied to the recycling of RIM polyurethane auto parts. For example, car door panels and dashboards can be made by adding about 6% RIM polyurethane powder and 15% glass fiber.

3. Use as filler

Polyurethane foam can be made into fine particles through low-temperature pulverization or grinding process, and the dispersion of this kind of particles is added to polyol for the manufacture of polyurethane foam or other products, which not only allows the recycling of waste polyurethane materials, but also effectively reduces product cost. The content of crushed powder in MDI-based cold-cured flexible polyurethane foam is limited to less than 15%, and up to 25% of crushed powder can be added to TDI-based thermally cured foam.

 

Chemical Recycling

The chemical recycling method is to use alcoholysis, amine hydrolysis, hydrolysis or pyrolysis methods to decompose polyurethane foam into polyurethane raw materials or other chemical raw materials.

1. Dihydric alcoholysis

The glycolysis method is the most widely used chemical recovery method. In the presence of small molecule glycols (such as ethylene glycol, propylene glycol, diethylene glycol) and catalysts (tertiary amines, alcohol amines or organometallic compounds), polyurethane (foam, elastomer, RIM products, etc.) Alcoholysis is carried out at a temperature of about 200°C, and the reaction is several hours to obtain renewable polyols. Recycled polyols can be mixed with fresh polyols to make polyurethane materials.

2. Amine solution

Polyurethane foam can be converted into the original soft polyol and rigid polyol through amination. The amination process is that the polyurethane foam reacts with amine during the pressurization and heating process. The amine disintegrant used includes dibutylamine, ethanolamine, lactam or lactam adduct, and the reaction can be carried out at a temperature below 150°C. The final product is a polyurethane foam prepared directly without purification, and can completely replace the polyurethane prepared with the original polyol.

3. Other chemical recycling methods

Hydrolysis method—Sodium hydroxide can be used as a hydrolysis catalyst to decompose soft and hard polyurethane foams to generate polyols and amine intermediates, which are used as raw materials for recycling.

Alkaline hydrolysis method—Polyether and alkali metal hydroxide are used as decomposers, and carbonates are removed after the foam is decomposed to obtain recycled polyols and aromatic diamines.

Pyrolysis—The soft polyurethane foam can be decomposed at high temperature under aerobic or anaerobic conditions to obtain oily substances, and polyols can be obtained by separation.

Energy recycling from polyurethane waste is a more environmentally friendly and economically valuable technology. The U.S. Polyurethane Recycling Committee is conducting an experiment to add 20% waste polyurethane soft foam to a solid waste incinerator.The results show that the burnt ash and emissions are still within the specified environmental protection requirements, and the heat energy released after the addition of waste foam has greatly saved the consumption of fossil fuels.European countries such as Sweden, Switzerland, Germany, and Denmark are also experimenting with technologies that use energy recovered during the incineration of polyurethane waste to provide electricity and heat for heating.

Since polyurethane is a nitrogen-containing polymer, no matter which combustion energy recovery method is used, the best combustion conditions must be used to reduce the formation of nitrogen oxides and amines. The combustion furnace needs to be equipped with appropriate exhaust gas treatment devices to reduce damage to the environment.

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